1. Field of the Invention
The present invention relates to microwave excited light sources which utilize a phosphor(s) or phosphor containing component(s) coated on or within components external to a microwave excited light bulb therein to produce a desired light output spectrum augmented by the light output spectrum produced by the phosphor(s) or phosphor containing component(s).
2. Description of the Prior Art
The Assignee of the present invention sells microwave excited light sources using light bulbs having a medium to high filling pressure and high applied microwave power which produce high radiance in the UV frequency range. Bulbs of a one to ten-inch nominal length are powered with a range of microwave power from 1 kW to 10 kW. These UV light sources have nominal power loads ranging from 100 watts/inch to 1000 watts/inch. The Assignee""s microwave excited UV light sources convert the input electrical power to a UV light output with an efficiency of between 10-35%. Microwave excited UV light sources have the advantage of producing high output power and a frequency stable spectrum from more than 3,000 hours of operation.
FIG. 1 illustrates a prior art UV spectrum produced by the Assignee""s microwave excited UV light sources. As is apparent in the UV range, there is a substantial drop off in output power between 300-350 nm. The spectrum illustrated in FIG. 1 is provided upon request to customers of the Assignee""s microwave-powered electrodeless lamps to enable the customers to best understand the frequency ranges present in the UV light output used for the customers"" UV light applications.
The aging of surfaces, coatings, etc., with irradiation from between 290-420 nm, is conventionally performed to determine the properties of the surface coatings in response to extended exposure to solar radiation. The higher the irradiance of the UV light, the more rapid an aging study may be completed. The spectrum of a commercial solar lamp has rising UV power emission in the spectral range between 300-350 nm. The prior art spectrum illustrated in FIG. 1 has a total maximum emission at about 330 nm. The Assignee""s microwave excited UV light sources can produce a much higher power irradiance than a commercial solar lamp and more efficiently convert the input power into light than a commercial solar lamp. However, the UV spectral distribution of the Assignee""s microwave excited UV light sources is not most suitable to perform solar aging studies in view of the large drop off in the potentially important wavelength range between 300-350 nm.
A need exists for a high efficiency, high power solar irradiation light source which simulates the UV light spectrum produced by the sun as well or better than standard solar lamps so as to permit accelerated solarization studies of a wide variety of surfaces, paints, coatings, etc.
In a fluorescent lamp, a phosphor placed on an inner wall of the lamp downshifts the UV emission of a low-pressure mercury discharge into the optical range. More than one phosphor or a phosphor with more than one activator may be used to produce a desired color.
Phosphors that fluoresce in low power lamps in the ultraviolet range between 295-400 nm produce UV-A, B or C emissions are used for tanning and medical treatment.
Phosphors containing thallium, lead or europium activators in a variety of host materials produce emissions which lie in the range between 300-350 nm. However, such materials are temperature sensitive and their light conversion efficiency decreases with temperature. The aforementioned properties restrict incorporation of these phosphor materials into a bulb wall with a temperature below 100xc2x0 C.
The present invention is a high efficiency, high intensity microwave driven light source having a preferred application as a UV light source. A light source in accordance with the invention utilizes high power microwave excitation to produce UV light with wavelengths which excite a phosphor(s) or phosphor(s) containing components or compositions coated on or within optical components external to the microwave excited light bulb. The phosphor(s) or phosphor(s) containing components or compositions produce light emissions in a desired frequency range(s) of the output spectrum which is additive to the power level of the output spectrum in the desired frequency range(s) produced by the microwave excited lamp bulb to achieve a desired power output in the desired frequency range(s) of or in the entire output spectrum. The downshifting provided by UV phosphor(s) or UV phosphor(s) containing components or compositions on surfaces of or within components of a microwave-powered UV light source external to the light bulb, whether on reflective surfaces, filters, windows, optics, or a pellicle, separates temperature sensitive phosphor(s) or phosphor(s) containing components or compositions from the high temperature of the microwave excited bulb so that the microwave powered light source can be operated at high power output with any desired light spectrum at whatever temperature is required for optimal operation. High intensity light produced by microwave excited light bulbs prevents phosphors from being coated thereon in view of their high output surface temperatures which may exceed 1,000xc2x0 C.
With the invention the spectral distribution of light produced by microwave-powered light sources, which are optimized for other purposes such as the efficiency of producing light from the input electrical power, permits operation without having to introduce additional chemical components or compounds, as dopants into the bulb fill.
As used herein, a phosphor(s) includes a phosphor(s) alone or as part of components or compositions containing a phosphor(s) which phosphoresce to produce light in the visible or UV range. The phosphors may be a surface coating on or within the optical components external to the light bulb.
A microwave excited light source in accordance with the invention includes a microwave source which produces microwaves; a microwave excited lamp bulb, coupled to the microwave source, which produces an output spectrum and operates within a first temperature range when producing an output spectrum with at least one frequency range of the output spectrum having a power level below a desired level; and an optical component, spaced from the bulb which operates in a second temperature range below the first temperature range, having at least one phosphor which is excited by another frequency range of the output spectrum, the at least one phosphor in response to the another frequency range outputs light in the at least one frequency range which increases the power level to the desired level. The optical component may be a filter through which the output spectrum passes. The optical component may be a reflector which reflects the output spectrum. The optical component may be a window through which the output spectrum passes. The at least one phosphor may be operational within the second temperature range and may be rendered non-operational at the first temperature range. The one and the another frequency range of the spectrum may be in the UV range.
The invention is a microwave excited UV light source including a microwave excited UV lamp bulb, coupled to the microwave source, which produces an UV output spectrum representative of UV light produced by the sun and having an operation temperature range when producing the UV output spectrum with at least one frequency range of the UV output spectrum in a first UV wavelength range having a power level below a desired level; an optical component, spaced from the bulb, which operates in a second temperature range below the first temperature range having at least one phosphor which is excited by at least one frequency range of the UV output spectrum within a second UV wavelength range shorter than the first UV wavelength range, the at least one phosphor in response to the at least one frequency range within the second UV wavelength range outputting UV light within the at least one frequency range of the first UV wavelength range which increases the power level to the desired level; and the at least one phosphor is operational within the second temperature range and is rendered non-operational at the first temperature range. The second UV wavelength range may have a maximum wavelength of approximately 300 nm; and the first UV wavelength range may be between approximately 300-450 nm and preferable, the first wavelength range may be between approximately 300-350 nm. The second wavelength range may be approximately centered about 250 nm. The at least one phosphor may be Ca3(PO4)2:Tl or (Ca0.9Zn0.1)3(PO4)2:Tl and have 3-4 mol % Tl. The at least one phosphor may be Sr2MgSi2O7:Pb, BaSi2O5:Pb, Ba1.6Sr0.4Si2O5:Pb, Ba2ZnSi2O7:Pb, or SrB4O7F:Eu. The optical component may be a reflector which reflects the UV output spectrum, a filter through which the UV output spectrum passes, or a window through which the UV output spectrum passes.